Why won't minecraft let me build on any servers?

Answer 1

Many servers have building rights which can only be given out by ops, admins, and/or mods (depending on setup). In order to play on these servers ask one of these ranked people to give you the right. Many servers have a test or amount of time you must be there before you can get building rights. If you keep searching many servers don't have building rights although most do.

Answer 2

It is more than likely that it is the owners of the servers themselves, not minecraft. Contact the server administrators and request building permissions.